[NFC][DirectoryWatchedTests] Unlocks mutexes before signaling condition variable

This should not affect actual behavior, but should pessimize the threading less
by avoiding the situation where:

  * mutex is still locked
  * T1 notifies on condition variable
  * T2 wakes to check mutex
  * T2 sees mutex is still locked
  * T2 waits
  * T1 unlocks mutex
  * T2 tries again, acquires mutex.

Differential Revision: https://reviews.llvm.org/D65708

llvm-svn: 367968